Snorkel Economy of St. Joseph, Missouri is an
OEM of self-propelled aerial work platforms that
lift workers and their tools high in the air to
perform tasks. Snorkel Economy's mobile lifters
are popular with construction companies,
aerospace manufacturers, and other companies that
need to place workers in hard to reach places.

A Snorkel Economy boom-type machine is
stabilised solely on its tires; it does not have
outriggers, special extensions or braces.
Balance, therefore, is one of the most important
specifications for the design engineers to
consider early on in the design cycle.

27 pin boom design

Recently, engineer Dave Engvall analysed an
articulated boom with 27 junctions known as pin
joints where the sections of the boom connect and
move, including a telescopic extension with a
basket on the end that serves as the work
platform. The boom had three main lift cylinders
and two other cylinders for levelling the basket.
Two aspects of Engvall's design were crucial: the
stability of the entire vehicle, and the load on
each one of the 27 pin joints.

"Our machines have to be stable in any
position of articulation," explained
Engvall, "so I needed to estimate the weight
of each component, and then place the lifting
structure in several different positions and
determine how much counterweight would be
necessary to maintain stability." To test
the feasibility of the design, Engvall imported
his CAD files using DXF translators into Working
Model, a dynamics/kinematics analysis software
package by Knowledge Revolution of San Mateo,
California. Working Model applies the laws of
physics to mechanical designs and simulates the
motions of objects. Working Model lets engineers
see the physical consequences of their designs in
graphic, animated form. Engvall could assign
specific values to his design, such as the
probable weight of each component, the forces
acting on the basket at different extensions, and
explore hypothetical situations such as placing
the vehicle on an incline. As a result, Engvall
could quickly abandon unrealistic models and
narrow his design to one that would work in the
real world.

Testing for instability

Engvall's first task was to find the most
unstable aspect of the new design. He focused his
attention on the lifting structure - consisting
of the boom, the engine that drives it, and the
counterweight that balances it - all set on a
turntable that rotates about a vertical axis in
the middle of the vehicle chassis. "I placed
my model in a known critical stability
orientation, and then I added weight to the
counterweight until I made the situation
stable," said Engvall. "It's not unlike
those scales used in a doctor's office, where the
idea is to balance the little metal weights until
they are level." The hardest balancing
problem for Engvall's design was side tip
stability, with the boom extended at a right
angle to the vehicle. The key point of balance or
"tipping fulcrum" in this position was
the tire.

"I modelled that situation with the tire
on one side of the machine, and then I used an
element in Working Model called a separator on
the opposite side, which constrains two objects
from getting closer together. They can get
further apart, but not closer." The
separator kept the tire (and, because they were
joined by a fixed point, the entire vehicle) from
intersecting the ground in order to keep the
model balanced. Engvall then ran the simulation.

In just a few seconds, Working Model
demonstrated at what point the mobile lifter was
stable. When the force on the separator was less
than zero, the machine tipped over. When it was
greater than zero, it didn't.

"It's visually interesting to
watch," Engvall noted, but the real
satisfaction was in knowing that the mathematical
data behind the simulation was highly accurate.
"I have great confidence in the
results," confirmed Engvall. "And it's
much more enjoyable to watch the machine tip over
in my mechanism model than in the real world.
Using Working Model has greatly streamlined the
calculations necessary to match the boom
structure weight with the counterweight."
Working Model's solved these calculations to an
80-bit number with a floating point that allowed
the calculations to be performed faster with
greater accuracy.

Getting the weight just right means Snorkel
Economy will be able to manufacture a machine
that not only meets ANSI safety standards, but
optimises the best type and weight of materials
to use to keep production costs from becoming
prohibitive.

"If you put more cost into a part, you
can save weight," explained Engvall,
"but there are a lot of trade-offs to
consider such as how expensive a material you
want to use to save weight." In Working
Model, the weight of each component can be
controlled, and if too much counterweight was
needed to make Engvall's design stable, then he
had to reduce the weight of some of the other
components.Engvall tested the stability of all
the pin joints, running his simulations over and
over to refine the ratio between material weight
and the optimum articulation of all the boom
joints and struts.

Meeting European safety
standards

Achieving side tip stability didn't mean
Engvall could proceed with his design. He had to
check his boom/counterweight balance combination
in other articulations, too. Snorkel Economy must
test for back tip stability (with the boom fully
extended overhead), and for external forces which
could act upon the basket, such as wind. Working
Model's Smart Editor feature allowed Engvall to
reposition his mobile lifter intact in any
configuration, quickly modify any physical forces
with its easy-to-use graphical user interface,
then run a new simulation in a matter of seconds.
Engvall could play back the simulations
frame-by-frame, in real time, or in a quicker
mode. Engvall even simulated different force and
balance specifications to meet European safety
requirements. Designing a mobile lifter that
requires fewer part substitutions to meet another
country's safety standards means Snorkel Economy
will save money making fewer modifications to its
vehicles for sales to foreign customers.

Measuring forces for FEA
analysis

Measuring the load stress or static force
balance on individual pin joints was the other
major application that Working Model helped
Engvall simulate. The weight of a pin affects the
load on all the pins, so if unacceptably high
stress is found to effect one pin, then that
junction will require a bigger pin, which in turn
increases the loads on the other pins.

"This is the simplest application of a
mechanism model, but it is extremely useful for
me," said Engvall. "If I did the static
force balance analysis myself, I would have to
describe a free body diagram of each component,
write equations for the translations and
rotational elements for each link, and solve them
all simultaneously. It's very time
consuming."

Instead, Engvall used Working Model to assign
the proper weight to each component, and then put
a pin joint at each location so the components
could move and then let the simulation run. This
determined the pin loads on all the joints. In
this way, Engvall was able to accurately measure
the forces for finite element analysis.

"The model doesn't have any idea how much
a pin can hold, so it just loads it up with
whatever it takes to achieve equilibrium,"
explained Engvall. After measuring the static
load on each of the pins, Working Model provided
the results that Engvall shared with a stress
analysis engineer, who then examined the design
of the connecting brackets in the boom using
finite element analysis to make sure they could
accept those loads.

Confidence in results

Using Working Model has saved Engvall
countless man-hours. It used to take more than a
week to create his models and do his static load
calculations, but now he can do both in a single
morning, and the simulations themselves take a
matter of seconds to complete.

"What I like about Working Model is that
I have confidence in the results," said
Engvall. "When I run simultaneous
simulations, the result is graphically clear and
reliable."

Using PC-based dynamic/kinematic analysis,
Snorkel Economy has leapfrogged one
time-consuming portion of its design analysis
cycle before prototyping a single part. The
ability to test the feasibility of his
articulated boom design in Working Model means
Engvall helps his company get its products to
market faster using the most cost effective
amount of raw materials.

"For what it costs and what it does,
Working Model is an astounding piece of
software," Engvall concluded.

Snorkel Economy is a 500+ employee company
located in St. Joseph, MO. They are one of the
leaders in the industrial mobile access market
(aerial work platforms). Snorkel Economy's
customers include large construction companies,
rental yards, and aerospace corporations. Snorkel
Economy can be reached by mailing to: